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Moreover, photosynthesis depends on several key factors that have already
been discussed and that could thus indirectly affect the occurrence of DOM
in natural waters (see also chapter Photosynthesis in Nature: A New Look ”).
On the other hand, declined concentrations of DOC have been observed in sev-
eral surface waters including south west of England, northern Scandanavia and
Italy (Worrall et al. 2004a , 2007 ; Schindler et al. 1996 ; Skjelkvåle et al. 2001 ;
Bertoni et al. 2010 ; Minella et al. 2011 ). Of the 315 catchments examined in the
UK, 18 % (55 catchments) have shown significant decreases in DOC concentra-
tion over the last 10 years (Worrall et al. 2007 ). DOC concentrations in the epilim-
nion have decreased from 119 to 57 μ M C (average values during 1980-1984 and
2000-2007, respectively). In lake Maggiore (Italy), chlorophyll a concentrations
averaged 5.9 μ g l 1 in the period 1980-1990, decreased to 4.0 μ g l 1 in the fol-
lowing decade (1990-2000) and underwent a further decrease (to 2.0 μ g l 1 ) in
the period 2000-2007 (Bertoni et al. 2010 ). The observed DOC decline is pre-
sumably caused by a decrease of total phosphorus and of the organic loadings to
the lake, because of a decrease of the anthropic impact. The consequences are a
decrease of in-lake productivity and pronounced changes in phytoplankton compo-
sition, including higher biodiversity, reduced biovolume and lower average com-
munity cell size (Bertoni et al. 1998 , 2008 ; Callieri and Piscia 2002 ; Morabito and
OggioniA 2003 ; Salmaso et al. 2003 ; Rogora 2007 ).
The decline of DOC concentrations in surface waters would be linked to lower
photosynthesis and often to relatively low contents of DOM. The latter may also
be the result of low precipitation, which generally decreases to input of soil alloch-
thonous DOM to natural waters. Waters with low contents of DOM would produce
low amounts of photoinduced or microbial end products, which may significantly
decrease the primary and secondary production with a subsequent decline of
autochthonous DOM (see also chapter Photosynthesis in Nature: A New Look ”).
Low production of autochthonous DOM would further contribute to the decline
of DOC in natural waters. In fact, production of autochthonous DOM may some-
times offset the DOM decomposition by natural sunlight. For the same reason, soil
inputs of allochthonous humic substances to surface waters during the summer
stratification period may enhance photosynthesis and increase the autochthonous
DOM in natural waters.
In some cases, drought can increase the DOM levels (Freeman et al. 2001a ;
Worrall and Burt 2008 ; Vazquez et al. 2011 ; Evans et al. 1999 ; Worrall et al. 2006 ;
Holden and Burt 2002 , 2003 ). The mechanism behind this phenomenon is that
waters with high contents of DOM may undergo high photoinduced and microbial
DOM degradation under drought conditions. The related production of photoin-
duced and microbial end products may be responsible for enhancement of high
photosynthesis and, therefore, of high primary and secondary production. The
latter phenomenon would ultimately lead to increased DOM contents.
The autochthonous production of DOM depends on several factors in natu-
ral waters, and particularly in lakes and oceans (Table 2 ) (Mostofa et al. 2005a ,
2009a ; Fu et al. 2010 ; Ogawa and Ogura 1992 ; Mitra et al. 2000 ; Yoshioka et al.
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